1. Field of the Invention
The present invention relates to methods and apparatus for a data transmission in a communication system, and more specifically, to methods and apparatus for improving performance of transmission with multiple code blocks and enabling fast decoding of transmissions with multiple code blocks in a communication system.
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) is a technology to multiplex data in frequency domain. Modulation symbols are carried on frequency sub-carriers and the sub-carriers overlap with each other in frequency domain. The orthogonality is, however, maintained at the sampling frequency in the assumption that the transmitter and receiver have perfect frequency synchronization. In the case of frequency offset due to an imperfect frequency synchronization or due to high mobility, the orthogonality of the sub-carriers at sampling frequencies is destroyed, resulting in Inter-Carrier-Interference (ICI).
A cylic prefix (CP) portion of the received signal is often corrupted by the previous Orthogonal Frequency Division Multiplexing (OFDM) symbol of multipath fading. When the cylic prefix (CP) portion is sufficiently long, the received Orthogonal Frequency Division Multiplexing (OFDM) symbol without a cylic prefix (CP) portion should only contain its own signal convoluted by the multipath fading channel. The main advantage of Orthogonal Frequency Division Multiplexing (OFDM) over other transmission schemes is that Orthogonal Frequency Division Multiplexing (OFDM) demonstrates robustness to compensate for multipath fading.
Single Carrier Frequency Division Multiple Access (SC-FDMA) that utilizes single carrier modulation and frequency domain equalization, is a technique that has similar performance and complexity to that of an Orthognal Frequency Division Multiplexing Access (OFDMA) system. Single Carrier Frequency Division Multiple Access (SC-FDMA) is selected as the uplink multiple access scheme in the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE). 3GPP LTE is a project within the Third Generation Partnership Project to improve the Universal Mobile Telecommunications System mobile phone standard to cope with future requirements.
Hybrid Automatic Repeat reQuestion (HARQ) is widely used in communication systems to combat decoding failure and improve reliability. N-channel synchronous Hybrid Automatic Repeat reQuestion (HARQ) is often used in wireless communication systems because of the simplicity of N-channel synchronous Hybrid Automatic Repeat reQuestion (HARQ). The synchronous Hybrid Automatic Repeat reQuestion (HARQ) has been accepted as the HARQ scheme for long term evolution (LTE) uplink in 3GPP. On the downlink of LTE systems, asynchronous adaptive HARQ has been accepted as the HARQ scheme due to its flexibility and additional performance benefits beyond synchronous HARQ.
Multiple antenna communication systems, which are often referred to as Multiple Input Multiple Output (MIMO) systems, are widely used in wireless communication to improve the performance of communication systems. In a MIMO system, a transmitter has multiple antennas capable of transmitting independent signals and a receiver is equipped with multiple receiving antennas. Many MIMO schemes are often used in an advanced wireless system.
When a channel is favorable, e.g., when the mobile speed is low, it is possible to use a closed-loop Multiple Input Multiple Output (MIMO) scheme to improve the system performance. In closed-loop MIMO systems, the receivers feed back to the transmitter the channel condition and/or preferred transmission MIMO processing schemes. The transmitter utilizes this feedback information, together with other considerations such as scheduling priority, data and resource availability, to jointly optimize the transmission scheme. A popular closed loop MIMO scheme is called MIMO preceding. With preceding, the transmit data streams are pre-multiplied by a preceding matrix before being passed on to the multiple transmit antennas.
Another perspective of a Multiple Input Multiple Output (MIMO) system is whether the multiple data streams for transmission are encoded separately or encoded together. All the layers for data transmission are encoded together in the Single Codeword (SCW) MIMO system, while all the layers may be encoded separately in the Multiple Codeword (MCW) MIMO system. Both Single User MIMO (SU-MIMO) and Multi-User MIMO (MU-MIMO) are adopted in the downlink of Long Term Evolution (LTE). MU-MIMO is also adopted in the uplink of Long Term Evolution (LTE), the adoption of SU-MIMO for Long Term Evolution (LTE) uplink, however, is still under discussion.
In a Long Term Evolution (LTE) system, when the transport block is large, the transport block is segmented into multiple code blocks so that multiple coded packets can be generated. This break-down of transport block provides benefits such as enabling parallel processing or pipeline implementation and flexible trade-off between power consumption and hardware complexity.
Different modulation schemes, such as Quadrature phase shift keying (QPSK), binary phase shift keying (BPSK), 8 Phase-shift keying (8-PSK), 16 Quadrature amplitude modulation (16-QAM), or 64 Quadrature amplitude modulation (64-QAM) may be used for adaptive modulation and for increasing the spectral efficiency of modulation. In case of 16-QAM modulation, quadruples of bits, b0b1b2b3, are mapped to complex-valued modulation symbols x=I+jQ. Different modulation positions, however, have different protection levels.
When multiple code blocks are transmitted, the performance of the transmission is dictated by the code block that has the worst performance. Channel interleaver, including mapping from coded bits of different code blocks to modulation symbols, and mapping from modulation symbols to time, frequency, and spatial resources, needs to be carefully designed to make sure that each code block gets roughly the same level of protection. When multiple code blocks are transmitted, it is beneficial to allow the receiver to start the decoding of some code blocks while the receiver is still demodulating modulation symbols for other code blocks. In a long term evolution (LTE) system, this presents a challenge because the channel estimation performance might be deleteriously impacted if there are not enough reference signals at the time of demodulation and decoding.
In order to maintain good channel estimation performance, interpolation of reference signals at selected resource elements located around a resource element to be estimated is often used to obtain channel estimation for the resource element with improved performance. This however, means that the demodulation of the modulation symbol in the resource element to be estimated needs to wait until all the resource elements selected for estimating the resource element are received. In other words, if the need for demodulation of the resource element to be estimated occurs before reception of the Orthogonal Frequency Division Multiplexing (OFDM) symbol which contains some of or all of the selected resource elements for estimating the resource element, the channel estimation performance for resource elements may be deleteriously affected.